In conventional electronically controlled fuel injecting device for internal combustion engines, the amount of fuel supplied to the engine would be increasingly corrected in consideration of deterioration of the fuel volatility during cold-engine operation or in consideration of a transiently increased amount of fuel adhered onto the inner wall surface of the intake passage (the increased flow rate of fuel flowing along the inner wall surface) during accelerating operation.
The requirement for increasing correction of fuel supply is different depending on the degree of the fuel volatility, that is, the degree to which the fuel is heavy or light. In case of heavier gravity fuel of a lower volatility, there is a great requirement for increasing correction of fuel supply. In general, it is difficult to perfectly maintain the degree of the fuel volatility, namely the degree to which the fuel is heavy or light, at a constant value. At least in case of heavy-gravity fuel used for the engine, in the conventional electronically controlled fuel injecting device, a characteristic necessary for the fuel-supply increasing correction suitable for heavy-gravity fuel would be preset, so as to satisfy the requirement for increasing correction of fuel supply.
However, under a condition in which the fuel-supply increasing-correction characteristic suitable for heavy-gravity fuel is held constant, if the characteristic for heavy-gravity fuel is applied to light-gravity fuel in case of which there is almost no necessity of increasing correction of fuel supply, there is a tendency of excessive increasing correction of fuel supply, thereby resulting in an excessively rich air-fuel ratio. As a result, unburned hydrocarbons (HC) contained in the exhaust gases tend to increase. Thus, it is desired that the level of increasing correction of fuel supply is suitably varied depending on the volatility of fuel supplied to the engine. For the reasons set forth above, it is necessary to detect a volatility (heavier-gravity or lighter-gravity) of fuel supplied to the engine.
One such technology, in which the level of increasing correction of fuel supply is corrected to a proper level based on the fuel volatility, has been disclosed in Japanese Patent Provisional Publication (Tokkai Heisei) No. 5-195840.
In the above-mentioned Publication, torque fluctuations in an internal combustion engine, resulting from surge, is monitored and the amount of increasing-correction of fuel is gradually decreased depending on the monitored torque fluctuations (surge torque fluctuations), and thus the increasing-correction amount is controlled so that the surge torque does not exceed an acceptable level. As a consequence, the increasing-correction amount of fuel is provided in a manner so as to conform to the fuel volatility.
However, in case of the method of fuel-supply correction described in the above-noted Publication, since the amount of increasing-correction is decreasingly adjusted gradually, it takes a long time until a desired increasing-correction level, serving as a parameter indicative of a fuel volatility, has been reached. Additionally, since the air-fuel ratio is forcibly varied through a feed-back control, i.e., a closed-loop control of an air-fuel ratio, in the event that the air-fuel ratio feed-back control begins, the fuel volatility cannot be detected during the feed-back control.
The prior-art apparatus suffers from the drawback that the level of increasing-correction of fuel supply, achieved during acceleration, cannot be satisfactorily in conformity with the fuel volatility, in the case that the air-fuel ratio feed-back control is executed soon just after the engine starts, by providing a heater nearby an O.sub.2 sensor used for detecting an air-fuel ratio, so as to improve exhaust-emission control characteristics, for example.
The present invention is made in view of the fact that a detected characteristic of a concentration of oxygen (O.sub.2) is affected by a concentration of hydrocarbon (HC). It is, therefore, in view of the above disadvantages, an object of the present invention to provide an apparatus which is capable of detecting a property of fuel, while executing a feed-back control for an air-fuel ratio.